A flotilla carrying humanitarian aid and activists, including Swedish climate campaigner Greta Thunberg, left Barcelona on Sunday vowing to try to "break the illegal siege of Gaza", organizers said.
Some 20 vessels set off from the port city on Spain's east coast just after 3.30 pm (1330 GMT) pledging to "open a humanitarian corridor and end the ongoing genocide of the Palestinian people", said the Global Sumud Flotilla -- sumud being the Arabic term for "resilience".
The group defines itself on its website as an independent organization with no affiliation to any government or political party.
The flotilla, flying Palestinian flags, has hundreds of people aboard, among them activists from dozens of countries including Irish actor Liam Cunningham and Spain's Eduard Fernandez.
Also aboard were European lawmakers and public figures including former Barcelona mayor Ada Colau.
The flotilla is expected to arrive at the war-ravaged coastal enclave in mid-September.
"The question here today is not why we are sailing. This story is not at all about the mission that we are about to embark upon," Thunberg told reporters.
"The story here is about Palestine. The story here is how people are being deliberately deprived of the very basic means to survive. The story here is how the world can be silent," she added.
For Cunningham, "the fact that you guys are here, and the flotilla is happening, is an indication of the world's failure to uphold international law and humanitarian law, and it is a shameful, shameful period in the history of our world. And we should be collectively ashamed."
Organizers said that dozens of other vessels are expected to leave Tunisian and other Mediterranean ports on September 4 to join the aid mission.
Activists will also stage simultaneous demonstrations and other protests in 44 countries "in solidarity with the Palestinian people", Thunberg, part of the flotilla's steering committee, wrote on Instagram.
"This will be the largest solidarity mission in history, with more people and more boats than all previous attempts combined," Brazilian activist Thiago Avila told journalists in Barcelona last week.
"We understand that this is a legal mission under international law," Portuguese lawmaker Mariana Mortagua, who will join the mission, told journalists in Lisbon last week.
Israel has already blocked two attempts by activists to deliver aid by ship to Gaza, in June and July.
In June, 12 activists on board the sailboat Madleen, from France, Germany, Brazil, Turkey, Sweden, Spain and the Netherlands were intercepted by Israeli forces 185 kilometers (115 miles) west of Gaza.
Its passengers, who included Thunberg, were detained and eventually expelled.
In July, 21 activists from 10 countries were intercepted as they tried to approach Gaza in another vessel, the Handala.
The Spanish government says it will "deploy all of its diplomatic and consular protection to protect our citizens" sailing with the flotilla, the country's Foreign Minister Jose Manuel Albares said Saturday.
Madrid last year recognized Palestine as an independent state.
The humanitarian situation in Gaza has worsened in recent weeks.
The United Nations declared a state of famine in the territory this month, warning that 500,000 people face "catastrophic" conditions.
The war in Gaza was triggered by an unprecedented cross-border attack by Palestinian group Hamas into Israel on October 7, 2023, which resulted in the death of 1,219 people, mostly civilians, according to an AFP tally based on official data.
Israel's retaliatory offensive has killed at least 63,371 Palestinians, most of them civilians, according to figures from the health ministry in Hamas-run Gaza. The UN considers those figures reliable.
